The present invention relates to a novel method for manufacturing a heat exchanging vehicle made of aluminum or an aluminum alloy, such as a heat exchanging fin, for use in an automobile or a home air conditioner, etc.
A heat exchanging fin made of aluminum or an aluminum alloy (hereinafter referred to simply as "aluminum") is generally manufactured, for instance, by forming a surface layer of organic or inorganic materials or composite materials mentioned above having good wettability over an aluminum sheet, pressing the aluminum sheet provided with such a hydrophilic surface layer to form a fin, and removing the lubricating oil such as a pressing oil attached to the fin surface during the pressing operation with a neutral or weak alkaline detergent aqueous solution, or by other means.
Recently, the fin pitch in heat exchanging units has been reduced for the improvement of heat exchanging efficiency and the miniaturization of heat exchangers. In an evaporator, such a reduced fin pitch causes the condensation of atmospheric moisture on fins, increasing resistance to an air stream, generating noises and blowing condensed water into a room. Further, it necessitates more energy for defrosting outdoors in winter time. As a result, it is extremely important that the fin surface have good wettability and high resistance to oil stain.
In addition, the fins should have a good corrosion resistance to white rust to prevent the formation of such a product which easily peels off.
To deal with these problems, the inventors have previously proposed a method for manufacturing a heat exchanging vehicle by forming a surface oxide layer with a weight of about 0.2-1.0 g/m.sup.2 over an aluminum sheet and forming it into a desired shape (Japanese Patent Laid-Open No. 58-106396). This proposal, however, is not completes satisfactory.
It has been found that when a precoated sheet is formed into a fin by pressing, degreased with an organic solvent such as a trichloroethylene to remove a lubricating oil which is attached during the pressing operation from a fin surface, and then assembled into an evaporator, the fin surface does not always have good wettability and high resistance to oil stain, depending upon degreasing conditions such as degreasing bath temperature and oil concentration in the degreasing bath, and the conditions of an inorganic surface layer such as an anodic oxidation layer and a hydrated layer, etc.
A fin sheet is usually obtained by annealing a so-called H1n-temper sheet of a predetermined thickness manufactured by cold rolling to a so-called H2n- or O-temper sheet having the desired mechanical properties.
Therefore, oil components remain on the aluminum sheet surface without being evaporated during annealing. These components become scorched, adhere to the surface and make it rough even after the formation of an hydrated surface layer. As a result, good wettability and high corrosion resistance to white rust are not necessarily obtained.